This invention relates generally to microwave ovens and more particularly to microwave oven apparatus for providing improved cooking uniformity.
As is known in the art, microwave ovens generally cook faster than conventional thermal ovens. However, heating uniformity has been a primary drawback or consideration throughout the development of microwave ovens. In first generation microwave ovens, the microwave energy was either excited in the cavity by direct insertion of the magentron probe, or the microwave energy was coupled to a waveguide that communicated with the cavity. In either case, the reflection of microwave energy from the cavity walls was considerable and complex standing waves were set up in the cavity causing "hot and cold spots." To improve heating uniformity, mode stirrers or moving metal parts were introduced into the cavity so that the complex standing wave patterns and hence the hot spots moved.
Heating uniformity within a food body is also a function of the geometry of the body, and it is particularly difficult to obtain uniform heating in certain geometries such as low profile pizzas or cakes. More specifically, the edge portions of a pizza tend to overcook while the center portion tends to undercook because of the characteristics of microwave absorption. Simply viewed, the entire pizza absorbs microwave energy propagating in the vertical direction; however, due to the depth of penetration of microwave energy in a food body, only the edge portions absorb microwave energy propagating in the horizontal direction.
More recent prior art microwave ovens have used radiating antennas or primary radiators which provide a directive substantially uniform pattern. With such antennas, a larger percentage of microwave energy is transmitted directly to and absorbed by the food without first reflecting from the cavity walls. Thus, while directive antennas have been used to concentrate more of the radiated energy to the center of a food body such as pizza, it is still difficult to provide uniform distribution of microwave energy across the entire pizza.